1. Field of the Invention
This invention relates to an objective optical system which is provided with two movable lens groups moving along the optical axis, and relates to an observation apparatus having the same.
2. Description of Related Art
Objective optical systems having known up to now include an objective optical system in which a lens group is moved to change a state of the objective optical system suitable for normal observation (which is called “normal observation state” below) into a state of the objective optical system suitable for close-up observation (which is called “close-up observation state” below) so that the objective optical system is closed to a particular object optionally selected from a plurality of objects present in an observation area by an observer to make it possible to observe the particular object in detail.
Such objective optical systems include, for example, an objective optical system: which consists of a first lens group with negative power, a second lens group with positive power, a third lens group with negative power, a fourth lens group with positive power, and a fifth lens group with positive power, in that order from the object side; and in which focusing is performed by moving the second and third lens groups along the optical axis in the range from the normal observation state to the middle state and by moving the fifth lens group along the optical axis in the range from the middle state to the close-up observation state (refer to Japanese Patent Kokai No. 2007-155887).
In addition, there is also an objective optical system: which consists of a first lens group with positive power, a second lens group with negative power, and a third lens group with positive power, in that order from the object side; and in which a magnification is changed and focusing is performed by moving the second lens group (refer to Japanese Patent Kokai No. 2007-233036).